By knowing the amount of sugar generated through photosynthesis, and
multiplying by 709 kcal/mole of sugar, we can calculate the amount of sunlight
which has been absorbed.

The rate of photosynthesis can be measured by analyzing the chemicals
unique to either side of the photosynthesis equation (e.g., CO2, O2, or
C6H12O6)

A. On Land. Place a plastic bag around a plant. Measure
the change in atmospheric concentration of CO2 during the daylight
hours. This will tell you NPP, as both photosynthesis and respiration
are taking place. Do the same at night, which will tell you the
respiration rate, as photosynthesis is not taking place. By adding these
two numbers together (NPP+respiration) you will know GPP

B. In Water. Collect a bucket of water. Mix it well and
measure the oxygen cencentration in the sample. Put it into two glass
gars, one of which has been wrapped in aluminum foil so that no light will
shine on it. Place the jars back into the water and let sit
for a few hours. Measure the Oxygen levels of the two jars. The
amount of Oxygen generated in the clear bottle will tell you NPP (both
photosynthesis and respiration taking place), while the amount of oxygen
used up in the shaded bottle will tell you Respiration. Add both to get
GPP.

C. Sugar generation. Place a plant into an air-tight plastic
bag. Pump radioactive CO2 into the bag, and let the plant briefly be
exposed to it. Take the plant out of the bag, grind it up, and determine
how much radioactive sugar has been created. This will tell you GPP
directly.

III. Estimating Primary Production Efficiency.

A. Primary production efficiency equals the fraction of sunlight
energy which falls on an area which ends up being harvested to make sugar.

B. GPP Efficiency is calculated by dividing GPP energy for a given
area by the total sunlight energy that area is exposed to.

For example:

7.08 moles of sugar are generated via GPP per year for each square meter
of Lake Mendota in Madison.

Multiplying this by 709, we calculate that 5017 kcal of sunlight is
absorbed during this process. By putting sunlight sensors out on the
lake, we can determine that 1,188,720 kcal of sunlight falls on each square
meter of Lake Mendota during that year.

GPP Efficiency for Lake Mendota = (5017 / 1,188,720) * 100 = 0.42%

GPP Efficiency for other ecosystems:

Forests 2 - 3½%
Grasslands 1 - 2% Crop Fields 1½%

C. NPP Efficiency is calculated by dividing NPP energy into total
sunlight energy.

Because Respiration losses are greater in some systems, NPP and GPP
efficiencies will not be the same. For the communities listed above, NPP
Efficiency equals approxil\matley 1%, even through GPP Efficiencies were up to
three times greater in some communities.

Thus, approximately 1% of sunlight is made available by plants for other
organisms to consume.

IV. Limits on Primary Production.

Primary Production is limited by many of the same factors previously
discussed.

On land, soil Nitrogen, water and light most often limit production

In water, Phosphorus and light most often limit production.

In aquatic systems, both too little sun, as well as too much sun, may
limit produciton.

Areas with too little sun (deep in water) are photolimited

Areas with too much sun (at water surface) are photoinhibited

Maximum pproduction occurs a few meters deep into the ocean, where
sunlight levels are not as high. These areas are photosaturated.